Despite its place as the centerpiece of evolutionary biology, fundamental questions remain about adaptive evolution that are difficult to address with the current set of examples of adaptations dissected to the molecular level. Instances of parallel evolution allow one to evaluate multiple outcomes of the process of adaptation under a common regime of natural selection. We propose to examine situations in which large assemblages of species experience parallel evolution as a tool to understand the genetic basis of specific adaptations and the dynamics of adaptation more generally. Specifically, we propose to focus will be on the medically important interaction between Na+,K+-ATPases, and their regulatory steroidal-glycosides, in the context of the natural genetic diversity underlying plant-herbivore (and predator-prey) conflicts. Our preliminary work reveals that diverse herbivorous insects specializing on steroidal-glycoside producing plants have repeatedly evolved insensitive versions of Na+,K+-ATPase in a surprisingly predictable manner. In particular, we discovered that when species only have a single copy of the gene, evolution appears to be largely limited to two functionally important amino acid residues. On the other hand, when a species has multiple copies of the gene, we found that these copies diverge in function and evolve tissue-specific expression patterns. Using a highly integrative suite of techniques (RNA and genome sequencing, computational biology, genome engineering as well as biochemical, physiological and behavioral assays), we will generate a large comparative genomics database to identify key amino acid changes underlying insensitivity of Na+,K+-ATPase to steroidal- glycosides and quantify their effects on organism performance. The resulting data and insights will comprise a major contribution to our understanding of adaptation and the origins of organismal complexity. In addition, given the medical importance of the Na+,K+-ATPase/steroid-glycoside interaction, our work will yield insights into the development of drugs to treat a number of Na+,K+-ATPase-associated neurological and physiological disorders in humans, as well as reduce the detrimental side-effects resulting from steroid-glycoside treatment of cardiac arrhythmias and congestive heart failure.